The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, three-piece or multi-layered golf balls. One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded thereon. One-piece balls are inexpensive and very durable, but typically do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid rubber core. These are the most popular types of balls in use today. In attempts to further modify the ball performance, especially in terms of the distance such balls travel, and the spin and the feel transmitted to the golfer through the club on striking the ball, the basic two piece ball construction has been further modified by the introduction of additional layers between the core and outer cover layer. If one additional layer is introduced between the core and outer cover layer a so called “three-piece ball” results, if two additional layers are introduced between the core and outer cover layer, a so called “four-piece ball” results, and so on.
Golf ball covers were previously made from balata rubber which was favored by some players because the softness of the cover allowed them to achieve spin rates sufficient to allow more precise control of ball direction and distance, particularly on shorter approach shots. However balata-covered balls, although exhibiting high spin and soft feel, were often deficient in terms of the durability of the cover which had a propensity to shear.
Accordingly, a variety of golf ball constructions have been developed in an attempt to provide spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance. This has resulted in the emergence of balls, which have a solid rubber core, an outer cover layer, and one or more so called intermediate layers, as well as the application of new materials to each of these components.
A material which has been often utilized in more modern golf balls includes the various ionomer resins developed in the mid-1960's, by E.I. DuPont de Nemours and Co., and sold under the trademark SURLYN®. These ionomer resins have, to a large extent, replaced balata as a golf ball cover stock material. More recent developments in the field have attempted to utilize the various types of ionomers, both singly and in blend compositions to optimize the often conflicting golf ball performance requirements of high C.O.R. and ball velocity, and cover durability, with the need for a ball to spin and have a so-called soft feel on shorter iron shots. However, the incorporation of more acid in the ionomer and/or increasing its degree of neutralization results in a material with increased polarity, and hence one which is often less compatible with other potential blend materials. Also increasing the acid content of the ionomer while increasing C.O.R. may render the ball too hard and brittle causing a loss of shot feel, control (i.e., the ability to spin the ball) and may render the cover too brittle and prone to premature failure. Finally, the incorporation of more acid in the ionomer and/or increasing its degree of neutralization typically results in an increase in melt viscosity which in turn greatly decreases the processability of these resins. Attempts to mediate these effects by adding softer terpolymeric ionomers to high acid ionomer compositions to adjust the hardness and improve the shot “feel” often result in concomitant loss of C.O.R. and hence distance.
In an attempt to improve on ionomer cover layers, nowadays most premium golf balls utilize polyurethanes or polyureas as materials to form the outer cover layer. Thermoplastic and thermoset polyurethanes both have been used in golf ball layers, and each provides for certain advantages. Polyurethane typically is formed as the reaction product of a diol or polyol, along with an isocyanate. The reaction also can incorporate a chain extender configured to harden the polyurethane formed by the reaction. Thermoplastic polyurethanes have generally linear molecular structures and incorporate crosslinking that can be reversibly broken at elevated temperatures. As a result, thermoplastic polyurethanes can be made to flow readily, as is required for injection molding processes. Because of their excellent flowability, thermoplastic polyurethanes can be positioned readily around a golf ball core using injection molding. Unfortunately, golf ball covers comprising thermoplastic polyurethane exhibit poor shear-cut resistance. Thus, while thermoplastic polyurethane covers are less expensive to make due to their superior processability and ability to be recycled they are not favored due to the resulting inferior ball performance.
In contrast, thermoset polyurethanes have generally networked structure that incorporate irreversible chemical crosslinking and exhibit high shear-cut resistance and is much more scuff- and cut-resistant than thermoplastic polyurethane. However, the irreversible crosslinks in the thermoset polyurethane structure make it unsuitable for use in injection molding processes conventionally used for thermoplastic materials as it does not flow freely, even when heated and waste material cannot be recycled back into the golf ball manufacturing process. Though they are more expensive to process than thermoplastic polyurethanes, thermoset polyurethanes have been used in golf ball layers. For instance, U.S. Pat. No. 6,132,324 to Hubert discloses a golf ball having a cover formed from thermoset polyurethane. The patent teaches a method for casting a thermoset polyurethane cover over an ionomer inner layer, including a step of measuring the viscosity “over time, so that the subsequent steps of filling each mold half, introducing the core into one half and closing the mold can be properly timed for accomplishing centering of the core cover halves fusion and overall uniformity.” The additional steps involved in casting a layer over those needed for injection molding the layer lead to added complexity and expense. Another patent discussing use of thermoset polyurethane is U.S. Pat. No. 6,435,987 to Dewanjee. This patent teaches thermosetting polyurethane comprising a toluene diisocyanate-based prepolymer, a second diisocyanate prepolymer, and a curing agent. Again, this method makes use of casting because the materials used would not be well suited to injection molding.
Another family polymeric material that has been the subject of attempts to adapt for use in golf ball layers are the polyamides such as nylon. These materials offer a number of potential advantages in golf ball layer formation given their well-known strength properties. Unfortunately many polyamides, including nylon, have proved to be too brittle for use in a golf ball cover. When efforts have been made in other fields to blend nylon with softer materials some degree of incompatibility often has resulted, rendering the blends susceptible to cracking and premature failure.
U.S. Pat. No. 4,690,981, the contents of which are incorporated herein by reference, shows soft terpolymer ionomers of ethylene/unsaturated carboxylic acid/softening comonomer which are useful in injection-molded items such as ski boots, ice skate shells, as coatings for fabrics, and as a replacement for balata in golf balls. The unsaturated carboxylic acid may be, for example, acrylic acid and methacrylic acid. The softening comonomer is, for example, an alkyl acrylate such as n-butyl acrylate. The 981 patent briefly mentions that the ionomers can be blended with other materials such as nylon, polypropylene, propylene-ethylene copolymers, linear polyethylene, and ethylene/unsaturated carboxylic acid copolymers. However, there is no indication that blends can be used for golf balls.
We have now surprisingly found that when such polyamide compositions are subjected to crosslinking via a variety of mechanisms the resulting polymers are extremely well suited to golf ball layer formation.
One such mechanism of crosslinking the polyamide compositions used in the present invention is disclosed in U.S. Pat. No. 4,671,355 which discloses a crosslinked nylon block copolymer, chemically, through the use of polyfunctional amine compounds. That is, the crosslinked nylon block copolymers are prepared by a reaction scheme in which polyfunctional amines act as crosslinking agents.
In Plast. Massy, 1993, No. 2, pp 35-37, which contains a paper entitled “Production and Properties of Crosslinked Compositions of Aliphatic Nylons” a study was conducted on the process of radiation crosslinking of an aliphatic polyamides The materials studied were nylon-6, nylon-6,6 and nylon-12. The polyfunctional monomers employed to accelerate crosslinking were triallyl cyanurate and triallyl isocyanurate.
Similarly, in the Chinese Journal of Polymer Science, Vol. 7, No. 1, there is a paper entitled “Characterization of Irradiated Crystalline Polymer-Isothermal Crystallization Kinetics of Radiation Induced Crosslinked Polyamide 1010”.
Finally, U.S. Pat. No. 6,099,416A discloses a non-ionomeric golf ball cover which has been treated with crosslink-inducing irradiation at levels of at least 2 megarads up to 15 megarads. The non-ionomeric resin cover is made from material selected from the group consisting of non-ionomeric acid copolymers and terpolymers, polyamides, styrene block copolymers, polyamide block copolymers and syndiotactic resins.
In view of known strength and durability properties of polyamides, it would be desirable to somehow utilize them in the construction of a golf ball. Specifically, it would be desirable to identify particular types of polyamide materials that might be uniquely adapted to serve as materials for golf ball construction. It would also be advantageous if the polyamide compositions could be injection molded given the processing advantages of such a method for golf ball layer formation.
We have now found that polyamide compositions suitable for golf ball layers and exhibiting improved durability may be prepared by mixing a polyamide with a crosslinking agent, which crosslinking agent includes peroxide based systems, or compounds having isocyanate functionality either as a simple diisocyanate (in blocked or unblocked form) or as a polyurethane or polyurea (also each in blocked or unblocked form) Other crosslinking agents include amine, and blocked amine, amide and blocked amide as well as crosslinking agents having polyol or glycidyl functionality.
We have also found that by selection of the correct crosslinking agent in combination with molding compositions, the golf ball layers may be formed by initially injection molding the polyamide/crosslinking agent followed by subsequent initiation of the crosslinking reaction by increasing the temperature over and above that used in the initial injection molding operation.